Method of enameling



Nov, 30, 1948.

METHOD J. C. ECKEL ETAL 0F ENAMELING Filed oct. 24, 1944 4 1, ff, l f

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uunranaluuuurnauaam vapnnunn/r INVENTORS Jos eph C. Ec/fe/ and HughE/Pom/'ne Patented Nov. 30, 1948 METHOD F ENABIELING Joseph C. Eckel,Ingram, and Hugh E. Romine, Grafton, Pa.

Application October 24, 1944, Serial No..560,183

4 Claims. l

`This invention relates to the art of enameling and, in particular, tothe coating of iron or steel' articleswith a finish layer of enamel,particularly white or light-colored vitreous. enamel, applied directlythereto without the application of a dark groundv coat as required bythe previous enameling practice.

It has proved impossible prior to our invention, to' enamel metalarticlesv with a nish layer of enamel by applying a single coat of fritdirectly to the base metal and ring it, because of the clevelopment ofbubbles,v pits or specks in the surface of the enamel. Although numerousattempts have been made to overcome the defects mentioned, none ofV themso far as We are aware has been successful. Niedringhaus Patent 200-626, granted in 1878, discloses a method of expelling the carbon fromthe base metal before coating with frit, by heating` it to a white heatin the presence of lime, butr this method has notachieved any commercialacceptance, so far as We are aware, probably because the treatmentdescribed does notv eliminate the iron carbide in the surf ace of themetal. Kautz Patent 2,099,340, granted more than half a century laterthanthe Niedringhaus et al. patent, describes the production ofv asurface layer of substantially pure iron on the formed article byoxidizing it and then reducing the oxide. This process,l however, hasnot been successful. We believe that they defect of` the process is thatit produces a coarse surface which traps the hydrates contained in thefrit, preventing the elimination of the water of crystallizationtherefrom in thedrying, withthe result that water vapor is evolved onring which is injurious to the' surface of the enamel.

Badger et al. (Ceramic Industry, July, 1937, p. 41) observed that theremoval of part of the surface carbon from a piece of high-carbon steel(drill rod, presumably about 1% carbon). permitted it to be successfullycoated with ordinary ground coat' enamell after it was heated in ahydrogen atmosphere to a temperature from 800' degrees toy 856 degreesC., but they did. not vdeal at all. with the problem of. applying nishenamels such asthel whiteenamel usedf'or refrigerators, bathroom xturesand the like, and

their disclosure did not enable the industryl to adopt the directapplicationV of commercial. coats.

of' finish enamels.

The practice in the art has therefore been to apply a groundcoat of thecobalt-oxide type [the ordinary ground coat enamel of Badger et all,before applying the finishl coat of" enamel, usually a white orlight-colored enamel. This practice involves greater processing time,laborV and material cost than would be required for applyinggaA singlecoatv of' finish enamel directly to the base. metal, but there has beenno other process satis.- factory to the trade at large for achieving thedesired results.

We have discovered a novel methodv of applying directly to the basemetal a single coat of finish enamelv and fusing it thereon without theoccurrence of pits or specks therein, and by which, despite the entireabsence of a ground,v coat, a nished product may be made superior tothat obtained by present practices,v with minimum rejections. Theproduct exhibits an improved adherence ofthe enamel to the base metaland less likelihood of chipping of the enamel because of the reducedthickness thereof. It also has a higher gloss.' The absence of surfacedefects eliminates the loss involved in the downgradingV and rejectionof ware made. by existing practice, which is necessaryA because of the.occurrence of such defects in a portion, at least, of the Ware.

Ina preferred practice, we form a sheet or' plate of low-carbon steel oriron enameling stock, i. e., steel (either open-hearth, Bessemer orelectric-furnace steel) containing from .010% to .25% carbon (althoughusually not over 0.10% carbon) into the shape of the desired nishedarticle. We then clean the article, and give it an acid pickle and anickel-sulphate dip. We then eliminate carbon from the surfaces of theformed` article by heating it to a temperature of 1100 degrees F. orabove in a decarburizing atmosphere and maintaining it at suchtemperature for a substantial period, i. e. from five to thirty minutes.After cooling the article substantially to atmospheric temperature, weapply directlls7 to the base metal` a layer of iinish coat vitreousenamel frit and heat. the article thusl coated `to a temperaturesufficient to cause fusion of the frit yand formation of a smoothcontinuous enamel layer.

A complete understanding oi the preferred practice of our invention maybe optained from the following. detailed description which refers`example.

out our method upon articles formed from any suitable enameling stock,e. g., low-carbon steel or iron sheets or plates having from .010% to.25% carbon. Figure 1 shows such a sheet designated 'I0, in theas-rolled condition. The sheet is 'of appropriate gauge, depending onthe nature 'of the article to be formed therefrom. The 'sheet is formedinto the shape of the desired in'ished 'article by known methods ofdrawing, bending, etc., together With such welding as may be required.Figure 2 illustrates an article II formed from the sheet Illa,speciflcally the food compartment of a mechanical refrigerator. We nextclean the article to remove grease used as a lubricant in forming, andperspiration spots resulting from the necessary manual handling, andsubject it to an acid pickle and Aa nickelsulphate dip, in the knownmanner, rinsing the article after each operation.

If the enamel to be applied contains a substantial quantity of a whiteor light-colored substitute for cobalt oxide, we believe thenickelsulphate dip may be eliminated. Compounds of the cerium group havebeen mentioned as a suitable substitute (German Patent 282,348). Otherelements whose compounds are mentioned in the literature for use inpromoting the adherence of white or light-colored enamels are antimony(Tetrick, Journal of the American Ceramic Society, p.,349, 1934),zirconium (Anon, Bureau of Standards, Technical News Bulletin No. 122,p. 10, 1927) and molybdenum (Kautz, Journal of the American CeramicSociety, p. 283, 1940). The nickel dip is necessary to improve theadherence of many white or colored enamel compositions (Andrews, Enamelsbook, p. 177, 1935). Cobalt solutions have been used in a similar manner(Anon, Journal of the American Ceramic Society, Abstracts, p. 519,1928). Tetrick, in the article cited, also discusses the value ofchemical etchingtreatments of the base metal in improving enameladherence. Hot acid gases may be used to etch the metal surface in amanner to improve enamel adherence generally (Turin, Iron Age, p. 70,1944). Etching should be done prior to the decarburizing operation. Webelieve that the most successful enamels for our process will be thosewhich have good adherence due to the enamel composition and thatcomplete reliance should not be placed on surface treatments such asnickel dipping and etching in order to obtain adherence as such surfacetreatments appear to give less satisfactory results in commercialpractice.

We then heat the article to a temperature of about 1100 degrees F. orabove, and preferably in the neighborhood of 1400 degrees F., in adecarburizing atmosphere, and hold it at such temperature for asubstantial period, say from ve to thirty minutes. Figure 3 shows onemode of performing this part of the process. A continuous or tunnel-typefurnace I2 is provided with a conveyor I3 for moving formed articlesprogressively therethrough. The furnace is provided with appropriateheating means to maintain the temperature at around 1400 degrees F. andthe speed of the'conveyor is so adjusted with reference to the length ofthe furnace that each article will be within the heating zone of thefurnace for the period of time indicated, i. e., from i'lve to thirtyminutes.

A decarburizing atmosphere is supplied to the interior of the furnace,through a conduit I4, for The composition of the gas may vary widely butit should include free hydrogenA and free water vapor 'in such amountsas to inhibit oxidation of the metal of the articles being processed.Gas of the following composition has proved satisfactory: 5% CO2, 10%CO, 12% H2, water vapor corresponding to a dew point of about 85 degreesF., and the balance substantially nitrogen. This composition of gas forthe atmosphere of the furnace I2 is similar to what is known asbright-annealing gas but has a somewhat higher water-vapor content.

The furnace I2 may conveniently have a heating-up zone, a, soaking zoneand a cooling zone. The above-mentioned atmosphere is preferablymaintained in each zone. Thus the articles are protected againstatmospheric oxidation until they have been cooled. The cooling zoneshould be so constructed and arranged as to reduce the temperature ofthe articles gradually to a nal value not greatly in excess of that ofthe atmosphere, unless a certain amount of oxide scale is deliberatelyproduced on the articles by removing them from the furnace attemperatures somewhat above atmospheric in order to provide theoxidation necessary when the articles are subsequently fired in aneutral atmosphere as is done in certain special enameling practices.

The dipping of the article in nickel sulphate may follow thedecarburlzing treatment just described instead of preceding it. It isimportant, however, that the pckling be done before the decarburizingoperation.

After the articles h'ave been processed as above,

described, they are sprayed or otherwise coated with a frit appropriatefor forming a nish layer of vitreous enamel, usually white orlight-colored containing an opacifler. This frit is applied directly tothe surface of the processed articles.

Figure 4 shows one of the articles at II a having.

a coating of flrit thereon.

The frit-coated articles are next fired in thev known manner by placingthem in a conventional enameling furnace such' as indicated at I5 inFigure 5. In this process, the ware is heated to a temperature of about1500 degrees F., i. e.,

slightly above the fusion point of the frit. On removal from the furnacethe ware is cooled in air, according to the usual practice in theenameling art. Theenamel layer thus formed on the ware is smooth andcontinuous, unmarred by defects, such as pits or specks. Figure 6illustrates to enlarged scale the layer of nish enamel I 6 applieddirectly to th'e base metal I1 and bonded therewith.

It will be apparent from the foregoing that our invention ischaracterized by important advantages over the previous practice ofenameling. The ability to apply a inish layer of enamel directly to thebase metal without the formation of defects in the surface saves thesubstantial cost and processing time involved in applying the groundcoat which has heretofore been necessary.

The preliminary processing of the formed articlesA Further advantagesVreside in the fact that the enamel layer` applied by our invention hasexcellent adherence to th'e base metal. In addition, the removal ofsurface carbides appears to improve the resistance of the ware tosagging under the,

temperature of the enameling furnace. Warping of the ware is alsoreduced or eliminated. This; appears to result'from thelrelief of`warping stress;

a'fordedby the heating of `the Ware for eli-mi- -natingfcarbides fromthe surface thereof. A. further advantage is that the total thickness ofenamel applied by our method is less than that applied-in th'e priorpractice. As a result, Ware enameled by our method is more resistant tochipping than that previously made. It also has a 'higher gloss. Whilethe invention has been described with reference to the use of low-carbonsteel as a base metal, it is also applicable to lowmetalloid steel andopen-hearth iron.

'We believe that the superior results obtained by .practicing our methodmay be explained as follows, although we do not limit ourselvesto thisparticular theory: The frit for forming a finish enamel such .as thewhite or. light-colored enamels used in bathroom fixtures,refrigerators, stoves,

and the.. like, must *include` an opaciiier. A typical formula is thefollowing:

Percent- Parts age Feldspar 3l. 5 33. 5 Borax 24. 4 25. 9 Quartz. 19.020.1 Soda ash. 4. l 4. 4 Soda nlitre.; 3. 5 3. 7 Fluorspar l1. 7 12.4

For use, this frit is milled with about 6% clay, an opacier, such aszirconium oxide type, in amount to produce desired opacity (generally 6to 8%) and the necessary amount of water. Frits of this type when fusednot Wet and bond themselves properly to th'e base except in the presenceof oxygen, free or combined. The milled frit is generally applied in aliquid vehicle over the surface of the article prior to heating andconsequently is in the form of a dull porous coating when applied. Thefrit is a glass and has no `fixed melting point, but as the temperatureis increased it does ultimately soften to the extent that the particlescoalesce. Until this coalescence occurs, the layer of frit is porous andthe oxygen in th'e furnace atmosphere can penetrate through to the base.If this base is naked iron or steel, gases form. If these gases escapethrough the coalesced frit of finish enamel, they leave pits or otherdefects. If they do not escape, they form bubbles or blisters.

In the ordinary enameling processes now in vogue, the ground-coat enamelis essentially a cobalt-oxide glass. A typical formula is the following:

For use, this frit is milled with about 6% clay. Such frit is relativelyquite fluid and has a low surface tension when fused by heating to usualenameling temperatures. We believe that the gases escape readily throughthis fluid ground coat and that any pits thus formed disappear as theenamel thereafter levels itself off. On the other hand, finish enamelshave a high viscosity because of the presence of the opacier, and ifpits form therein, the enamel will not level itself off to the extentnecessary to give a commercial iin- 'ish. If the nish enamel is appliedover a groundcoat enamel, there is no reaction as, for example,y'between the steel and the furnace atmosphere, which cause gases toevolve, since the steel base is walled on' by a continuous and unbrokenllayer of ground-coat enamel. We believe that the reactions which resultin gas formation in these circumstances are complex and that thecomposition of the gases may vary considerably under differentconditions. We believe, however, that theyare always at least in partcarbon monoxide.

The iron or steel principally used in the enameling industry varies incarbon content from .010% to 0.25%'. The lower figure is about theminimum obtainable by current steel-making processes in extensive useand producing metal in theform desired for fabrication. The particularcarbon content specied will depend upon a number offactors'such as thecomplex-ityof the articles and the corresponding difliculties of formingit, the stiffness desired in the nal product, etc. We believe that evenin the softest enameling iron or steel there is a substantial amount ofcarbon at or near the surface in the form ofiron carbide with which theoxygen from the furnace atmosphere will react to form carbon monoxide.In our process the treatment of the articles in the furnace I2 removesgas-forming compounds such as iron carbide from the surfaces of thearticles as well as inwardly of the surfaces to a predetermined depth,depending on the extent of the processing, the minimum objective being,in any case, to leave no carbides or other gas-forming constituents ator near the surface which would react with the furnace atmospherepenetrating through the porous frit during iiring.

It should be pointed out that even in the Dresent practice of rst firinga ground-coat enamel and then placing and firing a finish coatthereover, difficulties are sometimes encountered with what is known asre-boiling. This defect, which is Well known in the art, manifestsitself by the presence of pits or bubbles in the Ware, sometimesrequiring down-grading or outright rejection. Re-boiling defects mayoccur from even minor changes, many of which are extremely difficult tolocate. Re-boiling defects are entirely eliminated by our discoverybecause it permits the application directly to the base of a frit for afinish enamel, even though it contains substantial quantities of anopacier. Such enamels are so viscous at enameling-furnace that any gaseswhich might be given olf by the base metal would form blisters orbubbles which, if they break, would show as pits remaining in thesurface of the enamel after the gas escapes.

An important aspect of our invention is the surface treatment of thearticles after they have been formed to shape. We have found that if thetreatment is applied to the steel prior to fabrication, enamelingdefects are prone to occur in those parts of the article where the metalhas been bent or die-shaped to a material extent or has been abraded asin metal finishing and even in zones where the metal has been heated tohigh temperature as in welding operations which may be employed infabrication.

Although we have disclosed herein a preferred practice of our invention,it will be recognized that modifications thereof may be made within thescope of the appended claims.

We claim:

1. In a method of enameling in which an articlev is made `fromconventional ferrous enameling stock having a carbon content of from.010% to .25% and in which the article is approximately in its finalshape, the steps including, cleaning the article with a cleaningsolution and drying it to prepare the ferrous surface thereof forenameling, then heating the articleto a temperature in the neighborhoodof 1400 degrees F. in a decarburizing atmosphere containing freehydrogen and water vapor and effective to inhibit oxidation of and todecarburize the ferrous metal surface, maintaining the article at suchtemperature and in such atmosphere for a period of time between aboutand 30 minutes until its ferrous surface has been freed from gas-formingconstituents such as iron carbide, gradually cool- Ving the article in anon-oxidizingatmosphere substantially to atmospheric temperature,applying directly to the surface of the article a layer of light-coloredfinish-coat vitreous enamel frit containing an opacier, and heatingthearticle thus coated to a temperature sufficient to convert the frit intoa coat of fused enamel.

2. The method as defined by claim 1 characterized by saidfirst-mentioned atmosphere being composed substantially of 12% hydrogen,10% carbon monoxide, 5% carbon dioxide, water vapor corresponding to adew point of 85 degrees F. and the balance substantially nitrogen.

3. In a method of enameling in which an article is made fromconventional ferrous enameling stock having a carbon content of from.010% to .25% and in which the article is approximately in its finalshape, the steps including, cleaning the article with a cleaningsolution and drying it to prepare the ferrous surface thereof forenameling, then heating the article to a temperature in the neighborhoodof 1400 degrees F. in a decarburizing atmosphere containing freehydrogen and water vapor and effective to inhibit oxidation of and todecarburize the ferrous metal surface, maintaining the article at suchtemperature and in such atmosphere for a period of time between about 5and 30 minutes until its ferrous surface has been freed from gasformingconstituents such as iron carbide, gradually cooling the article in anon-oxidizing atmosphere substantially to atmospheric temperature,applying directly to the surface of the arthe article thus coated to atemperature sume.

cient to convert the frit into a ycoat of fused enamel.

4. In a method of enameling in which van article is made fromconventional ferrousy enameling stock having a carbon content of from,010% to .25% and in which the article is approximate- 1y in its finalshape, the steps including,:c1ean ing the article with a cleaningsolution and drying it to prepare the ferrous surface thereof forenameling, then heating the article to a temperature between about 1100degrees and about 1400 degrees F. in a decarburizing atmospherecontaining free hydrogen and water vapor and effective to inhibitoxidation of and toY decarburize the ferrous metal surface, maintainingthe article at such temperature and in such atmosphere for a period oftime between' about 5 and 30 minutes until its ferrous surface has beenfreed from gas-forming constituents such as iron carbide, graduallycooling the article in a non-oxidizing atmosphere substantially toatmospheric temperature, applying directly to the surface of the articlea layer of light-colored nish-coat vitreous enamel frit containing anopacier, and heating the article thus coated to a temperature suicientto convert the frit into a coat of fused enamel.

JOSEPH C. ECKEL. HUGH E. ROMINE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Badger et al.,

Aromic Industry, July 1937,

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